ref: c14d7f600d8c1d7c32221cf212f2f78acf970a5b
dir: /examples/dcv.peg/
Stmt = SPACE Expr EOL { printf("%d\n", pop()); }
| (!EOL .)* EOL { printf("error\n"); }
Expr = ID { var= yytext[0] } ASSIGN Sum { vars[var - 'a']= top(); }
| Sum
Sum = Product ( PLUS Product { int r= pop(), l= pop(); push(l + r); }
| MINUS Product { int r= pop(), l= pop(); push(l - r); }
)*
Product = Value ( TIMES Value { int r= pop(), l= pop(); push(l * r); }
| DIVIDE Value { int r= pop(), l= pop(); push(l / r); }
)*
Value = NUMBER { push(atoi(yytext)); }
| < ID > !ASSIGN { push(vars[yytext[0] - 'a']); }
| OPEN Expr CLOSE
NUMBER = < [0-9]+ > SPACE
ID = < [a-z] > SPACE
ASSIGN = '=' SPACE
PLUS = '+' SPACE
MINUS = '-' SPACE
TIMES = '*' SPACE
DIVIDE = '/' SPACE
OPEN = '(' SPACE
CLOSE = ')' SPACE
SPACE = [ \t]*
EOL = '\n' | '\r\n' | '\r' | ';'